The invention relates to a method and an apparatus for measuring the diameter of a monocrystal while drawing it from a crucible by determining the brightness profile at the melt/monocrystal change-over point and determining the diameter therefrom.
Extensive automation of the crystal-drawing process, often called the Czochralski process, is desirable. An essential factor in the automation is control of the crystal diameter which is of decisive importance to the use of the end product. However, a method for controlling the diameter of the crystal requires a very precise method of measurement. Several proposals have already been described in the literature for this. However, the known methods are mainly limited to measuring and controlling the substantially cylindrical part of a crystal, i.e. that part used for providing the end products (semi-conductor discs). In terms of weight, this constitutes by far the greatest proportion of the drawn monocrystal.
However, the drawn monocrystal also comprises other parts of considerably varying diameters and change-over zones between the different diameters which have a decisive effect upon the quality of the end product. First, starting with what is known as a crystal nucleus, a "neck" is formed by drawing, this having a diameter of between approximately 2 and 6 mm. Adjoining this neck is a substantially conical change-over zone extending to the required crystal diameter which, in present-day production, is between approximately 60 and 120 mm. This diameter should be maintained over the greatest possible length of the monocrystal, which length is on the order of approximately 1000 mm or more. The cylindrical portion of the monocrystal should be followed by what is known as an end cone. Given the above-mentioned prerequisites, it is not only necessary to have a very precise diameter program dependent upon the length of the crystal, but also necessary to have a continuous comparison involving precise measured values to enable the diameter adjustment satisfactorily. In principle, the diameter of the crystal can be influenced mainly by two adjustable values, namely, the drawing speed and the temperature of the melt in the crucible (bath temperature). To deal with this situation, a linked adjusting circuit is generally provided whereby, firstly, deviation in diameter is eliminated by altering the drawing speed and, secondly, after a predetermined difference in speed is exceeded, adjustment of the bath temperature is carried out. However, in the absence of precise measuring methods, no readily usable proposals regarding the fully automatic control of such process have yet been put forward.
In the related field of zonal fusing and zonal drawing, British Patent Specifications Nos. 986,293 and 986,943 disclose the idea of detecting the molten zone between the two parts of the crystal horizontally from the side with a television camera, analyzing the individual lines of the video signal image for increased brightness, and in dependence upon that line of increased brightness having the greatest radial spacing from the axis of the crystal, adjusting the stretch/upset ratio of the two crystal parts and therefore the diameter.
In arriving at the measured value, the brightness of the light radiated from the molten zone against the dark background plays an important part. Direct application of this system to the crucible-drawing method is therefore not possible since, with the television camera taking pictures in a horizontal direction, the edge of the melting crucible gets in the way and masks the melt/monocrystal change-over point within the crucible.
It is also known to use, in the crucible-drawing method, a television camera which, when taking pictures, is directed obliquely downwards toward the mouth of the crucible at a sharp angle to the axis of the crystal. However, measuring methods that make use of a television camera have the disadvantage of limited resolution because of non-linear deflection of the beam (in the television camera) such that errors of 1% of the measured diameter occur. By means of a very costly linearization system it has become possible to reduce the error to 0.5%. A disadvantage of this, however, is that the television camera tube cannot be replaced at the end of its service life without troublesome readjustment. Furthermore, the analysis of the signals resulting from taking a picture, that is recorded as a perspective view, is relatively complicated.
It is also known, from DE-PS No. 16 19 967, to aim a radiation detector obliquely downwards at a small surface area of the melt at the light-dark boundary of the diameter of the crystal. The radiation detector is then laterally adjusted with two micrometer screws arranged at right angles to each other and servo-motors according to a program of the desired variation in the diameter of the monocrystal. Although the accuracy in adjustment is relatively great, it is achieved only at the considerable expense of the precision mechanisms and control technique involved. Also in this system, the level of the bath of molten material in the crucible (which affects the crystal diameter as described above) has an effect which cannot be readily offset. This is due to the necessarily inclined viewing direction of the radiation detector. In addition, the field of view of this measuring arrangement is too limited.
The optical measuring method involving a viewing or picture-taking direction that is downwardly inclined at a sharp angle to the axis of the crystal based on the discovery that a clearly detectable light fringe is formed at the melt/monocrystal change-over point. The light fringe is attrituble to the radiation of heat from the melt and possibly from the inner wall of the crucible. Because of the surface tension of the melt and the wetting of the crystal, there is formed at this point a kind of throat which, although not having a temperature higher than that of the surrounding zone, nevertheless acts as a concentrating reflector from the adjacent radiating surfaces. This is seen by the observer and/or the camera equipment as the above-mentioned annular and narrowly limited light fringe which, against the other surfaces, has a clearly detectable additional intensity.